1. Field of the Invention
The present invention relates to a photoreceptor, for example, an electrophotographic photoreceptor.
2. Description of the Prior Art
Electrophotographic photoreceptor, for example a selenium photoreceptor or a selenium photoreceptor doped with arsenic, tellurium, antimony, etc., or a photoreceptor with zinc oxide or cadmium sulfide dispersed in binder resin have so far been known. However, these photoreceptors pose problems of environmental pollution, thermal instability and insufficient mechanical strength.
Meanwhile, an amorphous silicon (hereinafter abbreviated "a-Si") based electrophotographic photoreceptor has been proposed in recent years. The a-Si has so-called dangling bonds where the Si-Si bond is broken. This type of defect generates many localized energy levels in the energy gap. Therefore, there occurs hopping conduction of thermally excited carriers to lower the dark resistance while trapping of photo excited carriers by the localized energy levels results in poor photoconductivity. It is thus a practice to compensate for these defects by using hydrogen atoms to bond to silicon atoms to fill the dangling bonds.
The above hydrogenated amorphous silicon (hereinafter called "a-Si:H") exhibits a resistivity of 10.sup.8 to 10.sup.9 ohm-cm in the dark, which is about ten thousand times lower than amorphous selenium. Therefore, the photoreceptor comprising a single layer of a-Si:H has such a problem that its surface potential decays in the dark at a high rate and its initial charging potential is low. On the other hand, for the sensitive layer of the photoreceptor, this material has a very favorable characteristic that its resistivity greatly decreases when it is exposed to light of the visible or infra-red spectral region.
To endow such a-Si:H with the potential retention, it can be doped with boron to increase its resistivity as high as 10.sup.12 ohm-cm. However, it is not easy to control the boron doping level with satisfactory accuracy. Further, a resistivity as high as 10.sup.13 ohm-cm can be attained by introducing a trace of oxygen with boron. When used for the photoreceptor, however, this kind of material exhibits an inferior photosensitivity causing problems, such as unsharp potential drop at the edge and nonnegligible residual potential after exposure.
In addition, the photoreceptor with a-Si:H exposed in the surface has not yet been fully examined for the chemical stability of its surface, as for example, possible effects of long-term exposure to atmosphere or moisture or those of chemical species as generated under corona discharges. It is known, however, that after having been left to stand for more than a month it is affected so much by moisture that there is a remarkable lowering in the charging potential.
On the other hand, a method of manufacturing hydrogenated amorphous silicon carbide (hereinafter called "a-SiC:H") is described in Phil. Mag., Vol 35 (1978), etc. It is known that this material has characteristics, such as high heat resistance, high surface hardness, higher dark resistivity (10.sup.12 to 10.sup.13 ohm-cm) than a-Si:H, and optical energy gap changeable between 1.6 and 2.8 eV depending on the carbon content.
An electrophotographic photoreceptor comprising a combination of a-SiC:H and a-Si:H was proposed, for example, in the Japanese Patent Publication Open to Public Inspection (hereinafter called "Japanese Patent 0.P.I. Publication) No. 17952/1982. According to this invention, a triple layered photoreceptor is formed wherein an a-Si:H layer provides a sensitive (photoconductive) layer with a first a-SiC:H layer formed on its light receiving surface and second a-SiC:H layer on its back surface (on substrate electrode side).
It is noted however that this known photoreceptor has not yet been examined so extensively on how its characteristics may be affected by the thickness of individual layers. There was noted that the preferred thickness of the above first a-SiC:H layer was between 2,000 .ANG. and 5,000 .ANG.. However, a study made by the present author revealed that the above range of thickness provides a product that exhibits very poor performances in the sensitivity characteristics as the photoreceptor.
For the above known photoreceptor, there has been made almost no detailed examination on how the thickness of the first a-SiC:H layer may affect the characteristics of the photoreceptor. After full examination, the present author found a range of thickness that could never be expected from the prior art, which led to discovery of a type of photoreceptor of very excellent characteristics. In addition, examination was made on the thickness of the second a-SiC:H layer and also a-Si:H layer to find a range of thickness for individual layers that could improve performance.